Multi-layer polymeric films must in general be possessive of several qualities in order to be useful in the various packaging industries, particularly where it is desired to employ the films to food product usage. In general, the films must have a high degree of clarity and gloss in order to impart a positive cosmetic appearance to the food product packaged, must have a high degree of resistance to tearing and/or puncture as measured by the dart impact test, should have a high resistance to deformation at elevated temperatures in order to assist in processibility and storage in various environments, should have sufficient tensile strength, tensile yield, hot tack strength, moisture impermeability, a wide heat seal range, high seal strength, good low temperature sealability, relatively low density, and should preferably be completely recyclable. From the recyclibility standpoint, it is highly desirable that the films be free from chlorine, and are comprised of materials compatible with a range of polymers.
The most popular materials conventionally employed as films in the packaging industry have included linear polyethylenes, high density polyethylenes, high molecular weight high density polyethylenes, ethylene vinylacetate copolymers, polyvinylchloride (PVC), isotactic and syndiotactic polypropylenes and interpolymers of the aforesaid either alone or in combination with one another as in, for example a polymer blend in either single-layer or multiple-layer structural arrangements.
The use of polypropylene as a multi-layer film material has been known for quite some time. The films are favorable from several standpoints, particularly of strength, scratch-resistance, and optical properties. However, the films exhibit very poor heat-sealing properties. To obviate the disadvantages inherent in polypropylene, outer skin or sealing layers comprising typically, for example, random copolymers of propylene with other alpha olefin monomers, are coextruded onto one or both sides of the base layer. However, these sealing layers generally have the disadvantage that often impair the optical properties of the film composite as a whole. Additionally, the higher melting points of the outer sealing layers can create difficulty when the temperature of the heat sealing device is set to the melting point of the outer layer insomuch as the excessive heat causes the inner layer(s) to pit or severely deform thus causing weakness along the seal. Alternatively, difficulty is seen when the heat sealing temperature is set for the melting point of the inner layers in that the outer layers do not become sufficiently fused to form a homogeneous seal. Machines for forming seals on polyolefin films typically have forming collars or bars which shape and hold a flat piece of film in a desired position and actuate hot metal sealing bars or jaws to contact the film, thus causing a seal to be formed. It is important that the outside of the film has a higher melting temperature than the inside of the film so that when the jaw re-opens or the bar is removed, the outside of the film which has not melted is not stuck to the sealing jaw or bar. When using multi-layer films the temperatures which must be applied for sealing are so high that it is difficult to achieve the proper balance of heat necessary between the layers, and severe shrinkage in the sealing zone is often a resultant problem. The finished packages have a less than optimal appearance and an inferior sealed-seam strength.
Another difficulty often encountered by pioneering film products intended for use in packaging foodstuffs are the regulations set forth by the US Food and Drug Administration (FDA) at 21 C.F.R. 177.1520 (d)(iii) and (d)(iv) related to the content of the films which are soluble in xylene and n-hexane.
Accordingly, it is an object of this invention to provide a flexible film product comprising only polymers of propylene wherein the sealing temperatures of the outer layers are close enough to that of the core layer to avoid processing difficulties affecting seal integrity.
It is a further object of this invention to provide a multi-layer film product having superior optical properties over similar films of prior art.
It is a further object to provide film products having the aforesaid qualities which is also suitable for use in the foodstuff packaging industries.
It is a further object of this invention to provide a multi-layered film structure which is free from chlorine.
It is a further object still of this invention to provide a multi-layered film product which can meet high heat resistance specifications required for high-speed processing of film products.
It is a further object of this invention to provide a multi-layered film product which is sufficiently tear and abrasion resistant to withstand conditions normally experienced by film products in the packing industry and end uses, and which meets FDA regulations for use with foodstuffs. by film products in the packing industry and end uses, and which meets FDA regulations for use with foodstuffs.